Attention has been paid recently to a power source for an automobile, such as an electric vehicle, a hybrid car, etc., and a lithium ion secondary battery and a lithium ion capacitor for idling stop.
As an electrolytic solution of a lithium secondary battery, a nonaqueous electrolytic solution in which an electrolyte, such as LiPF6, LiBF4, etc. is dissolved in a cyclic carbonate, such as ethylene carbonate, propylene carbonate, etc., and a linear carbonate, such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc., is used.
In order to improve battery characteristics of such a lithium secondary battery, such as load characteristics, cycle property, etc., various investigations regarding a nonaqueous solvent or an electrolyte salt to be used in such a nonaqueous electrolytic solution have been made.
NPL 1 proposes a complex salt having, as a ligand, an ether compound having a specified structure and reports that hard volatility is exhibited, and also proposes that the foregoing ether complex salt is formed into an electrolytic solution and utilized.
PTL 1 discloses an electrolytic solution using a complex electrolyte of a glyme, such as triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), etc., with an alkali metal salt, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc., and a secondary battery and describes that thermal stability is high.
PTL 2 discloses a nonaqueous electrolytic solution secondary battery using a nonaqueous electrolytic solution containing lithium difluorophosphate and describes that the storage property is improved.
PTL 3 describes that an electrolytic solution containing a compound having a skeleton including a hetero element, the compound being a liquid at 25° C. and having a dielectric constant of 5 or more and a viscosity rate of 0.6 cP or less, such as dimethoxyethane, diethoxyethane, acetonitrile, etc., as well as lithium difluorophosphate (LiPO2F2) suppresses degradation of battery characteristics at the time of high-temperature storage.
In PTL 4, a nonaqueous electrolytic solution including lithium difluorophosphate is disclosed, and in Example 3 thereof, an example in which 4.6% by mass of lithium difluorophosphate was added is described.